The present disclosure relates to a participating station for a bus system and a method for data transmission in a bus system.
Driving systems are currently being developed in which vehicles travel on public roads autonomously or without human intervention in individual steering, braking or acceleration procedures. During this development, increasingly influential driving assistance functions are being installed in the vehicles for completely autonomous driving of the vehicles. The driving assistance functions control individual driving situations in a targeted manner, such as e.g. parking, stop-and-go, freeway driving, etc., and must be reliably available. This imposes stringent requirements on the data transmission rate between the control devices.
The CAN bus system, for example, can be used for communication between sensors and control devices in vehicles, in particular automobiles. With the CAN bus system, messages are transmitted by means of the CAN and/or CAN FD protocol as described in the current ISO-CD-11898-1 as the CAN protocol specification with CAN FD.
The data transmission becomes even more reliable if a further bus is installed along with the established CAN bus. Signals can then be transmitted redundantly via both bus lines, as in the case of FlexRay, for example. In the case of a redundant installation of this type, an electrical fault in one of the two buses does not result in complete communication failure.
For example, it is possible to connect four control units redundantly via two CAN buses as participating devices or participating stations of the bus system in order to transmit the signals redundantly between the control units. It is also possible here for three further control units to be connected to one of the two redundant CAN buses. As a result, these three further control units can similarly participate, albeit not redundantly, in the communication.
A bus architecture of this type has weaknesses, particularly if a large number of long spur lines are installed to the control units as participating stations. This is the case, for example, if the participating stations are connected to the bus from a central point, such as a switch, in a star configuration. Long spur lines generate reflections and reduce the maximum transmission rates, particularly in the case where CAN FD is used. In order to avoid long spur lines, both bus lines are frequently looped through, i.e. into the control unit, from there to the CAN participating station and are then fed out once more. As a result, more connection pins are required on the connector.
A further disadvantage arises from the fact that, according to the current ISO-CD-11898-1 as the CAN protocol specification with CAN FD, following the arbitration for the following part of the bus message, a completely different modulation can be used in order to increase the data rate. For example, a switchover to the Ethernet Physical Layer is possible following the arbitration. However, spur lines are unsuitable due to the reflections, particularly for transmission by means of amplitude modulation at a high transmission rate.
A further disadvantage consists in the fact that messages intended for individual bus participants only are also always picked up by all other participating stations, thereby also allowing unwanted interception and manipulation. This must be regarded as critical, particularly in the case of external devices.